Supercapacitors are useful for many applications because of their high power density, long cycle life and the potential applications on both military and commercial devices. For example, supercapacitors are important to the designs of portable laser systems and electric vehicles. Two mechanisms are associated with energy storage in a supercapacitor, namely electrical double layer charge storage and pseudo-capacitance charge storage. The capacitance of the former comes from the charge accumulation at the electrode/electrolyte interface, and therefore highly depends on the pore structure of the electrode, including such parameters as pore size and accessible surface area to the electrolyte molecules. The latter capacitance mechanism arises from to the fast reversible faradic transitions (electrosorption or surface redox reactions) of the electro-active species of the electrode, including surface functional groups, transition metal oxides and conducting polymers and this type of supercapacitor is also called electrochemical supercapacitors. The pseudo-capacitance from reversible faradic reactions of an electro-active material offers a higher power storage capacity than the electrical double layer capacitance mechanism.
Transition metal oxides have typically been considered to have a great potential to increase the capacitance in the electrochemical supercapacitors. Amorphous hydrated RuO2 has attracted particular interest as a supercapacitor electrode material with a capacitance over 700 F/g having been achieved, significantly higher than that has been observed with an electrical double layer capacitor. Unfortunately, hydrated RuO2 is too rare and expensive to be commercially viable as a supercapacitor material. Supercapicitors utilizing nano-crystalline vanadium nitride materials have exhibited capacitance of 1340 F/g at a 2 mV/s scan rate, which is far more than that of the hydrated RuO2 based supercapacitors. Such a high capacitance is believed to be caused by a series of reversible redox reactions on few atomic layers of vanadium oxide on the surface of the underlying nitride nanocrystals, which exhibit a metallic electronic conductivity (σbulk=1.67×106Ω−1 m−1).
Thus, there remains a need to supercapacitor material having even higher capacitance and using more readily available materials. The present invention addresses this need.